This invention relates to rotating machine components and to a method of repairing such components, particularly turbine rotors, via welding.
Steam turbine rotor materials, for example, typically are not selected on the basis of their weldability. Rather, materials are identified and selected specifically to provide a highly reliable rotating element that will withstand the normal conditions that such equipment experiences during plant operations. With advances in welding/metallurgical technology, however, the option of rotor weld repairs and turbine rotor upgrades are viable solutions for an equipment user. Of course, no weld repair or upgrade should be undertaken without proper controls, a full understanding of the design intent, and the material type or types involved in the fabrication.
Rotor weld repairs are performed for many reasons. Some of these reasons are convenience, maintenance of interchangeability, cost, and the need to bring the machine back on line in a timely manner. It is also possible to redesign using existing rotor forgings, when there is an understanding of the basic design parameters of the machine. Upgrading or repairing in this manner, if carried out in accordance with a precise methodology, can result in a less costly alternative to new parts.
Several problems are encountered in welding rotating elements. The two principal problems relate to distortion and material property changes. Either of these factors will adversely effect the reliable operation of, for example, a steam turbine rotor. It is therefore important to understand that certain turbine rotor materials and associated parts are more weldable than others, and that the selection of the welding option not only depends upon the type of material to be used in the welding process, but also upon the conditions to which the elements to be welded will be subjected in service.
Numerous attempts to provide a workable process for turbine rotor welding have been documented in the patent literature. For example, see U.S. Pat. Nos. 4,958,431; 4,940,390; 4,903,888; and 4,897,519 which describe repair procedures for worn and/or damaged surfaces of turbine components where the components are rotated during the welding process. A post-weld heat treatment operation may be provided as a stress relief measure.
U.S. Pat. No. 4,884,326 discloses a method and apparatus for repairing and resurfacing turbine liner walls during rotation of the turbine shaft and turbine blades.
U.S. Pat. No. 4,237,361 discloses a method of building up layers of metal on a region of a workpiece by fusion arc welding. The patent specifically discloses reconditioning a shaft for a turbine rotor by building up the surface of a shaft while the latter is rotated under a main welding head.
The attempts at providing a successful rotor welding repair procedure to date have been less than completely satisfactory, particularly in light of the many considerations involved in obtaining a successful weld repair. For example, a steam turbine rotor, although a single rotating element, is subjected to many complex stresses. Changes in the design parameters can have a very significant impact on the service life as well as short term reliability of the turbine. For example, steam turbine rotors, by design, are subjected to various lateral and torsional vibration levels, and changes in the shaft mass center, uniform stiffness and configuration generally can increase and/or change the sensitivity response to shaft vibration levels. These changes can dramatically effect operating stresses which can, in turn, affect the ability to operate at full load, and can also result in substantial reduction and fatigue life.
Another major factor in rotor welding, apart from mechanical integrity, is the concern for rotor stability. A rotor that is dynamically unstable as a result of welding is as much a failure of the weld process as is poor mechanical properties in the weld material. Dynamic instability is a condition where an unbalance or shift in rotor mass away from the shaft centerline has occurred during the welding operation. This shift can result from shaft distortion which, in turn, can result from uneven heat distribution in the shaft during welding or during post-weld heat treatment processes. Distortion can also result from non-uniform stress distribution which has caused plastic yielding of the shaft material. Ultimately, distortion affects the shaft in some form of dynamic instability. This instability is the key component in any rotational fatigue type failure.
In order to control possible shifting of mass or distortion in a shaft, the understanding of the influences of welding and post-weld heat treatment processes must be obtained first. Welding processes impart a combination of compressive and tensile stresses into the base material. As weld material cools, it contracts and hoop stresses also occur in the base metal. The weld also puts the base metal fibers into tension. These tensile and compressive stresses are not uniform in the base metal due to lack of uniformity in each weld pass. To counter these stresses, it is necessary to weld in such a manner as to reduce stresses, such as, for example, by employing a temper bead weld technique. In accordance with the technique, weld beads are overlapped in such a way as to temper the previous weld pass, thus subsequently reducing the overall stresses due to levels welding.
Although most of the weld stresses can be reduced by welding techniques, not all such stresses can be eliminated. It is therefore necessary to perform post-weld heat treatment (PWHT) on each weld repair to remove any residual stresses remaining in the weld. Distortion can also occur in PWHT processes, however, as a result of uneven heating or poorly configured heating elements. This distortion is brought about by a sag or bend in the shaft due to uneven axial and radial growth during heating.
In order to counter these effects, a heat treatment process in accordance with this invention can be utilized where the shaft is continuously rotated about its longitudinal axis not only through preheat, welding, and PWHT, but also through subsequent cooling to room temperature.
In this process, described for exemplary purposes in conjunction with a steam turbine rotor, the rotor is placed in a horizontal position and, after machining to remove the defect, is continually rotated about its longitudinal axis or centerline during preheat and welding. Heating under these conditions will greatly reduce the possibility of rotor sag. With the addition of load compensating centers, the shaft can grow axially without restraint.
The process also involves local PWHT through the use of high-frequency induction (HFI) heating. In this way, a shaft may be heated in a uniform manner so as to avoid distortion due to uneven axial and radial growth. HFI heating process also allows for a variety of heating methods. For example, HFI can be utilized in a case-hardening manner so as to stress relieve an inlay/overlay type weld. In this method, the entire shaft is not heated, since the only area that needs stress relieving is the welded surface. This reduces distortion by minimizing heat input to the shaft and allows for a constant shaft rotation. For example, HFI heating may also be used in otherwise conventional heat treatment processes. HFI may be used, for example, to heat the shaft, so that heat is allowed to conduct through the shaft, thereby stress relieving larger welded areas. Again, HFI allows for controlled and even heat input and constant shaft rotation, all key factors for distortion control.
Another feature of this invention is the continuation of shaft rotation after welding during PWHT and throughout the cooling of the rotor to room temperature, to further insure uniform distribution of remaining stresses in the weld material and in the base metal. Machining to final dimensions may then be carried out in accordance with required specifications.
Thus, in accordance with its broader aspects, a process for welding a rotatable machine component is provided which includes the steps of:
a) rotating the component about a longitudinal axis of rotation thereof;
b) preheating an area of the component to be welded;
c) depositing a plurality of weld beads in the area;
d) post weld heat treating the area; and
e) cooling the area to room temperature;
wherein steps b) through e) are carried out during continuous rotation of the component.
It is a further preferred feature of the invention that the welding step be carried out by a submerged arc welding process, and that the preheating and post weld heat treating operations be carried out by high frequency induction heating.
Additional objects and advantages will become apparent from the detailed description which follows.